Magnetic storage media, such as hard disks, store information in the form of magnetization of a magnetic material. A write head creates a magnetic field that magnetizes the magnetic material over a small area. A bit of information is represented by a property of the magnetized area, such as the direction of magnetization, for example. Magnetization in one direction may represent a binary “1” while magnetization in a different direction may represent a binary “0”. A read head detects the value of the stored bit by sensing such differences in the magnetization.
A protective layer, or overcoat, may be used to protect the magnetic material from mechanical damage and chemical corrosion. Without such a protective layer, the magnetic material may be damaged, and information lost, if, for example, a head crashes into the magnetic material or if the material oxidizes from exposure to air and moisture.
There is a constant effort to increase the information storage density of magnetic recording media by decreasing the area needed to store a single bit of information. As this area is decreased, however, numerous challenges arise. Because of the geometry and physics of magnetic recording, as this storage area is made smaller, the thickness of the protective layer must be decreased while its mechanical and chemical integrity are maintained. In addition, as the storage area is made smaller, the energy required to change the magnetization of the magnetic material may decrease to a point where thermal agitation at room temperature is enough to disrupt the magnetization, and the stored information is lost. This problem may be avoided by increasing the coercivity of the material—essentially the strength of magnetic field needed to magnetize the material and store a bit. The problem is, for the smaller desired storage areas, a write head may not be able to produce the required larger magnetic field magnitudes while still functioning as needed to store information.
A proposed solution to these challenges is heat-assisted magnetic recording (HAMR). In this technique, the small area of the magnetic material in which information is to be stored is momentarily heated to a temperature well above room temperature, such as 150 degrees Celsius (° C.) or higher. This may be done, for example, with a focused pulsed laser as a heat source. While the storage area is maintained at this elevated temperature the magnetizing field is applied and the material is magnetized so as to represent a desired bit value. Once the bit is stored, the heat source is turned off. The material then rapidly cools back to room temperature and the magnetization is “locked in”, thus storing the bit value.
The use of HAMR presents new challenges for the protective layer. The material of the protective layer must withstand the elevated temperatures used in HAMR without losing its mechanical and chemical integrity needed to protect the magnetic material. At the same time, the protective layer must be thin enough to allow robust, durable magnetic storage. Materials currently in use for protective layers in magnetic storage media, such as hydrogenated amorphous carbon (a-C:H) do not meet these requirements.